The present invention relates to the making and treatment of metallic workpieces, particularly to improvements in a method of preparing blanks which can be converted into the vanes of impellers, diffusers and analogous components of centrifugal pumps or like flow machines. Still more particularly, the invention relates to improvements in a method of reinforcing one or more marginal portions or edge portions of a substantially or partially flat workpiece or blank so as to render the marginal portion more resistant to corrosion, cavitation and analogous deleterious influences. The method can be resorted to with particular advantage in connection with the treatment of those portions of metallic blanks which are to constitute the inlet edges or suction edges of vanes in the impeller of a centrifugal pump.
At the present time, the inlet edges and/or other edges of vanes for use in the impeller of a suction pump are reinforced or armored in the following way: A raw piece of casting is treated in the region of the edge or edges by removing some of its material so that the treated portion has a predetermined thickness. The thickness of the thus treated portion is thereupon increased by cladding to form two layers of armoring material. In the next step, the surplus of the armoring material is removed by one or more chip- or shaving-removing tools to thereby impart to the treated portion the final outline or shape. In the next step, the just discussed portion of the blank is subjected to a heat treatment.
The above outlined conventional procedure exhibits a number of serious drawbacks, especially if the treatment involves those portions of a blank which are hard to reach when the blank is converted into the vane or blade of an impeller or the like. Thus, the conventional procedure does not ensure that the thickness of the armor which is applied to the selected portion or portions of the blank is uniform throughout or that such thickness varies in accordance with a predetermined pattern, the armoring material is subject to lamination, it can develop so-called penetration notches as a result of the application of armoring material by cladding, and the outlet edge or discharge edge of the vane is likely to develop heat-induced cracks. Moreover, the reinforced edge is simply incapable of standing the corrosive or other destructive influences which develop in a centrifugal pump or a like flow machine. The aforementioned heat treatment entails a certain distortion of the blank; in the case of blanks which are to be converted into or which are to constitute the vanes or blades of impellers or the like, the distortion can be in the range of one, two or even three millimeters which is highly undesirable because it interferes with the assembly of the distorted part with other parts of the machine. Still further, and since the armoring operation involves the application of metallic material by welding, the armoring material is likely to penetrate between the blank for the vane and the customary copper plate which is interposed between the blank and the bath.
Since the outlet or discharge edge of a vane is not accessible, its finishing treatment normally involves so-called blind grinding. In fact, in most instances repair work at the side of the outlet edge is not possible at all and must be carried out at the inlet side.